This invention relates generally to a nonwoven web having fine fibers and a small pore size distribution and a method for forming such a web. The method of the present invention uses a reactor granule resin having an initial broad molecular weight distribution which resin has been modified to narrow its molecular weight distribution and to increase its melt flow rate. Consequently the nonwoven web can be formed by melt-blowing at high throughputs. Such nonwoven webs are particularly useful as barrier layers for fabric laminates.
Nonwoven fabric laminates are useful for a wide variety of applications. Such nonwoven fabric laminates are useful for wipers, towels, industrial garments, medical garments, medical drapes, and the like. In heavier basis weights the laminates are used in recreational applications such as tents and as car covers. Disposable fabric laminates have achieved especially widespread use in hospital operating rooms for drapes, gowns, towels, footcovers, sterilization wraps, and the like. Such surgical fabric laminates are generally spunbonded/meltblown/spunbonded (SMS) laminates consisting of nonwoven outer layers of spunbonded polyolefins and an interior barrier layer of meltblown polyolefins. Particularly, Kimberly-Clark Corporation, the assignee of the present invention, has for a number of years manufactured and sold SMS nonwoven surgical fabric laminates, sterilization wrap and recreational fabrics under the marks Spunguard.RTM. and Evolution.RTM.. Such SMS fabric laminates have outside spunbonded layers which are durable and an internal meltblown barrier layer which is porous but which, in combination with the spunbond layers, inhibits the strikethrough of fluids or the penetration of bacteria from the outside of the fabric laminate to the inside. In order for such a medical fabric to perform properly, it is necessary that the meltblown barrier layer have a fiber size and a pore size distribution that assures breathability of the fabric while at the same time inhibiting strikethrough of fluids and bacteria.
The current meltblown web used in the manufacture of the Kimberly-Clark Evolution.RTM. medical fabric laminate has pore sizes distributed predominantly in the range from 10 to 15 microns with the peak of the pore size distribution greater than 10 microns. While such a meltblown web has advantages as a barrier layer, significant improvement in porosity and inhibition of strikethrough can be achieved with a meltblown web having average fiber sizes of from 1 to 3 microns and having a distribution of pore sizes so that the majority of pores are in the range of 7 to 12 microns with the peak of the pore size distribution less than 10 microns. More particularly, improved performance characteristics with respect to porosity and strikethrough can be achieved when the meltblown web has pore sizes distributed predominantly in the range from 7 to 12 microns, with a lesser amount of pores from 12 to 25 microns, and with virtually no pores greater than 25 microns as measured by the Coulter Porometer.
It is therefore an object of the present invention to provide a nonwoven web for use as a barrier layer in a fabric laminate which nonwoven web has an average fiber diameter of from 1 to 3 microns and pore sizes distributed predominantly in the range from 7 to 12 microns, with a lesser amount of pores from 12 to 25 microns, with virtually no pores greater than 25 microns, and with the peak of the pore size distribution less than 10 microns.
It is likewise an object of the present invention to provide a nonwoven fabric laminate having a barrier layer of fine fibers and small pore size distribution such that the resulting fabric laminate has pore sizes distributed predominantly in the range from 5 to 10 microns, with a lesser amount of pores from 10 to 15 microns, with virtually no pores greater than 22 microns, and with the pore size distribution shifted downward from the pore size distribution of laminate structures made using conventional meltblown webs.
The foregoing objectives are preferably obtained by forming a meltblown web from a propylene polymer resin having a broad molecular weight distribution and having a high melt flow rate which resin is modified by the addition of a small amount of peroxide prior to processing to achieve an even higher melt flow rate (lower viscosity). In general, the present invention involves starting with a propylene polymer in the form of reactor granules which polymer has a molecular weight distribution of 3.6 to 4.8 Mw/Mn, preferably 3.6 to 4.0 Mw/Mn and an initial melt flow rate of about 400 gms/10 min to 3000 gms/10 min at 230.degree. C. Such a molecular weight reactor granule polymer is then modified to reduce and narrow the polymer's molecular weight distribution to a range from 2.2 to 3.5 Mw/Mn by the addition of up to 3000 parts per million (ppm) of peroxide. During the meltblowing process, the modified reactor granule polymer has an increased melt flow rate from 400 gms/10 min. to 3000, for example, to a range between 800 up to 5000 gms/10 min at 230.degree. C.
Particularly preferred embodiments include a polypropylene resin in the form of a reactor granule having a starting molecular weight distribution of 3.6 to 4.8 Mw/Mn and an initial melt flow rate of from 600 to 3000 gms/10 min. at 230.degree. C. which is combined with a small amount of peroxide, less than 500 ppm, to produce a modified polypropylene having a very high melt flow rate of up to 5000 gms/10 min. at 230.degree. C. and a narrower molecular weight distribution of 2.8 to 3.5 Mw/Mn.
Alternatively, an improved meltblown web for use as a barrier layer can be formed by utilizing a resin, particularly polypropylene, having a narrow molecular weight distribution and having a lower melt flow rate which resin is modified by the addition of a larger amount of peroxide prior to meltblowing to achieve a high melt flow rate. The starting reactor granule polypropylene resin in this case has a molecular weight distribution between 4.0 and 4.8 Mw/Mn and a melt flow rate ranging from 400 to 1000 gms/10 min. at 230.degree. C. The polypropylene resin is modified by adding peroxide in amounts ranging from 500 to 3000 ppm (the higher amounts of peroxide being used in connection with the lower initial melt flow rate). The modified polypropylene resin has a melt flow rate, up to about 3000 gms/10 min. at 230.degree. C. and a narrow molecular weight distribution of 2.2 to 2.8 Mw/Mn, for example.
Most preferably, the starting polypropylene resin for the meltblown web of the present invention is a polypropylene reactor granule which resin has a molecular weight distribution between 3.6 and 4.8 Mw/Mn, has a melt flow rate of up to 3000 gms/10 min. at 230.degree. C., and is treated with about 500 ppm of peroxide to produce a modified resin having a melt flow rate greater than 2000 gms/10 min. at 230.degree. C. and a molecular weight distribution of from 2.8 to 3.5 Mw/Mn. The broader molecular weight distribution at the high melt flow rate helps minimize production of lint and polymer droplets (shot).
Other objects and advantages of the invention will become apparent upon reading the following detailed description and upon reference to the drawings.